Method for injecting plastic material

ABSTRACT

A method for injecting plastic material or another injectable material into an injection molding tool comprises the following steps: plasticizing the plastic or other injectable material in a plasticizing apparatus ( 2 ); transporting the plasticized plastic or the plasticized other injectable material into a dosing apparatus ( 3 ), which has a fluidic connection ( 4 ) to the plasticizer apparatus ( 2 ), and specifically in essentially precisely the quantity that is to be introduced into the injection molding tool ( 1 ); transporting the plasticized plastic or the plasticized other injectable material, essentially in that quantity which is to be introduced into the injection molding tool ( 1 ), from the metering apparatus ( 3 ) into an injection apparatus ( 5 ), via a fluidic connection ( 6 ), while at the same time preventing the plastic or other injectable material from backflowing from the metering apparatus ( 3 ) into the plasticizing apparatus ( 2 ); injecting the entire quantity of plastic or the entire quantity of another injectable material, which is situated in the injection apparatus ( 5 ), into the injection molding tool ( 1 ), while at the same time preventing the plastic or the other injectable material from flowing from the injection apparatus ( 5 ) back into the metering apparatus ( 3 ). In this way the production of small and ultra-small injection molded parts is facilitated, and the injection of a reproducibly precise quantity of melt is accomplished.

BACKGROUND OF THE INVENTION

In the process of injection molding plastics, plasticizers and injectionapparatuses are customarily used. Typically, a plasticizer and aninjection molding screw plasticize plastic material. The melt is theninjected into an injection molding tool by an axial movement of thescrew. To prevent the melt from backflowing during the injectionprocess, a non-return valve is usually disposed at the end of the screw.

Recently, the demand for dimensionally small injection-molded parts hasincreased. Common examples of such parts include micro-mechanicalcomponents (e.g., micro gear wheels for watches), medical-technicalhardware, and optoelectronic elements (e.g., parts for opticalwaveguides).

SUMMARY OF THE INVENTION

The production of these small part encounters problems when manufacturedwith classical injection molding machines, however. The problem relatesto the small amounts of hot thermoplastic that must be injected into theusually cold mold. This necessarily results in a critical thermaltransition region in the end region of the screw or in the nozzleregion.

The present invention is directed to avoiding this thermal transitionregion or minimizing its effect, especially when injection molding smallinjection molded parts, to achieve precise metering of the quantity ofinjection-molding material, such as plastic, which is injected.Specifically, the inventive method and an apparatus can produce anddeliver even the smallest quantities of melt to yield small and evenultra-small molded parts by precisely controlling the dose. The pressurebuildup here preferably occurs as close to the cavity as possible.

In general, according to one aspect, the invention features a method forinjecting plastic material or other injectable material into aninjection molding tool. The method comprises plasticizing or melting theplastic or other injectable material in a plasticizing, or similar,apparatus. The material is then transported into a dosing. or metering,apparatus, which has a fluidic connection to the plasticizer apparatus.Preferably, the amount of material is essentially precisely the quantitythat is to be introduced into the injection molding tool. The materialis then transported, essentially in that quantity which is to beintroduced into the injection molding tool, from the metering apparatusinto an injection apparatus, via a fluidic connection, while at the sametime the material is prevented from flowing from the metering apparatusback into the plasticizing apparatus. The entire quantity injectablematerial, which is situated in the injection apparatus, is injected intothe injection molding tool, while at the same time the material isprevented from flowing from the injection apparatus back into themetering apparatus.

Preferably, provision is here made that the injectable material that issituated in the injection apparatus is temperature-stabilized in such away that it remains a liquid melt.

In general, according to another aspect, the invention also features anapparatus for injecting plastic material or other injectable materialinto an injection molding tool. The apparatus comprises a plasticizingapparatus, in which plastic material or another injectable material isplasticized. A fluidic connection is provided between the plasticizingapparatus and a metering apparatus. The metering apparatus functions totake-up a prescribed quantity of injectable material. Preferably, anelement, such as a non-return valve, is disposed in the connection, toprevent the injectable material from backflowing. A fluidic connectionis also provided between the metering apparatus and an injectionapparatus. The injection apparatus receives the injectable material thatis to be injected into the molding tool before it is injected. Thequantity of injectable material that is transported into the injectionapparatus essentially corresponds to the quantity that is to beintroduced into the injection molding tool, in the preferred embodiment.Finally, means is provided to prevent the injectable material fromflowing from the injection apparatus back into the metering apparatus.

In the preferred embodiment, the means that prevents the injectablematerial from flowing from the injection apparatus back into themetering apparatus is implemented by an injection piston, which blocksthe fluidic connection between the metering apparatus and the injectionapparatus during the injection process.

Also, preferably, the means to prevent the injectable material fromflowing from the injection apparatus back into the metering apparatus isimplemented using an injection piston, which blocks the fluidicconnection between the metering apparatus and the injection apparatusduring the injection process.

Finally, the injection apparatus is ideally surrounded bytemperature-stabilizing heaters, for example, at least in part, whichprevents solidification of the melt.

In summary, the inventive method and design advantageously achieve apressure buildup in the melt during the injection process that occursvery close to the cavity. This results in a higher quality injectionmolding process, since the method and apparatus make it possible tometer and measure exactly the injected plastic, while at the same timereducing or controlling the thermal transition.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 schematically shows a cross-section of a plasticizing andinjection apparatus for plastic or other injectable material, in a firststage of the process, according to the invention;

FIG. 2 shows the same structure at a later time;

FIG. 3 shows the structure at a still later time;

FIG. 4 shows the structure at a still later time; and

FIG. 5 shows an alternative design of the plasticizing and injectionapparatus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an injection molding apparatus, which has been constructedaccording to the principles of the present invention, in its firstprocess stage.

An injection molding tool 1 has a cavity, into which melted plastic isto be injected, so as to produce a molded part. For this purpose,plastic granulate or powder is melted in a screw-plasticizer unit 2, inwell-known fashion. The plastic melt passes through a line 4 (a firstfluidic connection) into a metering apparatus 3. The metering apparatus3 essentially comprises a piston-cylinder unit, into which that quantityof plastic melt is introduced. The quantity of the melt is controlled asprecisely as possible, to be that which is needed to produce a moldedpart.

In one implementation, the apparatus is designed so that the motion ofthe piston of the metering apparatus 3 is detected such as by an encoderand supplied to a machine controller, and the plasticizing process inthe plasticizer unit 2 is interrupted by the controller as soon as thepiston has reached the desired position (taking into account thetemperature and pressure); the precise and required quantity of melt isthen in the metering unit 3. In every case, it is important that, beforeinitiating further steps, a precisely defined quantity of melt has beentaken in by the metering apparatus.

During the metering process, i.e., while the melt is introduced into themetering apparatus 3, the injection piston 10 of an injection apparatusis in such an advanced position that no melt flows through the line (asecond fluidic connection) 6 from the metering apparatus 3 into theinjection apparatus 5.

FIG. 2 shows the next stage of the process. Now, the injection piston10, which injects the melt into the mold 1, is pulled-back by thecontroller (see also the arrow in FIG. 1), so that the second fluidicconnection 6 between the metering apparatus 3 and the injectionapparatus 5 is enabled or established: This allows melt to flow from themetering apparatus 3 into the injection apparatus 5.

Backflow of the melt from the metering apparatus 3 into the plasticizingapparatus 2 is prevented by a non-return valve 8 in the first fluidicconnection 4 between the plasticizer apparatus 2 and the meteringapparatus 3.

Since the precise quantity of melt has been stored in the meteringapparatus 3 that is required to produce the molded part, activation ofthe piston of the metering apparatus 3 (see arrow in FIG. 2) causesexactly this quantity to be expelled, through the line 6, into thecylinder of the injection apparatus 5.

FIG. 3 shows a later stage of the process. The quantity of melt storedin the metering apparatus 3 has been completely expelled into theinjection apparatus 5. Consequently, the piston of the meteringapparatus 3 is now in its foremost position. The melt situated in theinjection apparatus 5, which preferably corresponds exactly to thequantity which is needed to produce a molded part, is then injected intothe mold 1 by the injection piston 10 of the injection apparatus 5 (seearrow in FIG. 3).

The geometric arrangement of the injection apparatus 5 and especially ofits piston 10 achieves the result that, when the melt is expelled fromthe injection apparatus 5 into the mold cavity, the melt is preventedfrom flowing back into the metering apparatus. After the connection 6has been passed, the piston actually blocks this connection and thenforms the means 9 to prevent the melt from flowing back in the preferredembodiment. Other techniques or systems, however, may be used for thispurpose such as controlling the relative pressures or non-return valves.

In one implementation, an injection apparatus comprises anpparatus toactuate an injection element as described in patent application Ser. No.09/291,398, filed on a common date, by Gruber, et al., which isincorporated herein in its entirety by this reference.

As FIG. 4 shows, the piston 10 of the injection apparatus 5 is pushedforward so far that it expels all the plastic material situated in theinjection apparatus 5. As further shown, preparation of the next shot isalready begun: The plasticizing apparatus 2 is already again producingplastic melt, which is expelled into the metering apparatus 3 (see arrowabove the metering apparatus 3).

As can further be seen in all the figures, heating elements ortemperature-stabilizing devices 7 are present, which surround at leastparts of the apparatus, to ensure that the melt cannot “freeze” in theapparatus or in its parts.

FIG. 5 shows another embodiment of the apparatus. In principle, thisapparatus operates precisely like the one shown in FIGS. 1 to 4.However, as can be observed, the plasticizing apparatus 2, the meteringapparatus 3, and the injection apparatus 5 are disposed at an angle toone another, and specifically in such a way that the three units 2, 3,and 5 work toward a common intersection point, which is situated at thepoint where the longitudinal axes of the three units meet.

For demolding the finished injection-molded parts from the mold or fromthe gate, the design can be such that the injection piston 10 executes ashort push motion, and thus takes over the function of an ejector.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims. For example, the inventivemethod and the inventive apparatus are also well suited to process otherinjectable materials, such as e.g. metal; the invention thus is notlimited to the processing of plastics.

What is claimed is:
 1. A method for injecting material into an injectionmolding tool, comprising: providing a plasticizer unit with alongitudinal axis, a metering apparatus with a longitudinal axis and aninjection apparatus with a longitudinal axis, each longitudinal axisbeing disposed at an angle to one another and meeting at a fluidicconnection; melting the material in the plasticizer unit; transportingthe material from the plasticizer unit into the metering apparatus viathe fluidic connection; metering in the metering apparatus a precisequantity to be introduced into the molding tool; transporting thematerial, in the quantity which is to be introduced into the moldingtool, from the metering apparatus into the injection apparatus, via saidfluidic connection, while at the same time preventing the material frombackflowing from the metering apparatus to the plasticizer unit, themetering apparatus having a uniform width in communication with andsubstantially all the way to the injection apparatus; and injecting thematerial, which is situated in the injection apparatus in the quantityto be introduced in the molding tool, into the molding tool, while atthe same time preventing the material from flowing from the injectionapparatus back into the metering apparatus.
 2. A method as described inclaim 1, wherein the material, situated in the injection apparatus, istemperature-stabilized to maintain a liquid state.
 3. A method asdescribed in claim 1, wherein the material is a plastic.
 4. A method asdescribed in claim 1, wherein the material is a thermoplastic.
 5. Amethod as described in claim 1, wherein the step of preventing thematerial from backflowing from the metering apparatus comprises passingthe material through a non-return valve.
 6. A method as described inclaim 1, wherein the step of preventing the material from flowing fromthe injection apparatus back into the metering apparatus comprisesblocking the fluidic connection between the injection apparatus and themetering apparatus with a piston of the metering apparatus.